Thermal instability, or in the case of fuels, fuel instability, generally refers to the formation of undesired deposits that occurs when hydrocarbon fluids, such as fuels, are subjected to high temperatures. In the case of fuels, it is generally accepted that there are two distinct mechanisms occurring within two overlapping temperature ranges. In the first mechanism, referred to as the coking process, a generally consistent increase in the rate of formation of coke deposits occurs up to about 1200.degree. F. (about 650.degree. C.). Coke formation is the result of high levels of hydrocarbon pyrolysis, and eventually limits the usefulness of the fuel. A second lower temperature mechanism generally peaks at about 700.degree. F. (about 370.degree. C.) and involves the formation of gum deposits. This second mechanism involves oxidation reactions that lead to polymerization which includes the formation of gums. Both coke and gum formation and deposits can occur simultaneously at temperatures where the above ranges overlap.
In the past, the solution to the problem of gum and coke formation was primarily directed toward placing limitations on fuel chemistry and impurities associated with fuels, as disclosed in U.S. Pat. Nos. 2,698,512, 2,959,915 and 3,173,247, which are incorporated herein by reference. However, even with the most elaborate special treatment of a fuel, coke formation cannot be entirely eliminated because, regardless of the purity of the hydrocarbon used, coke formation will occur if sufficient temperatures and durations are sustained. On the other hand, the chemistry of a hydrocarbon fluid and the chemistry of a vessel used to contain the fluid can have a major influence on deposit mechanisms and deposit rates at the temperatures where it is most desirable to use the fluid. In the lower temperature region where gum formation occurs, oxygen from air dissolved in the liquid is the major adverse ingredient. Boiling may amplify this adversity because of the oxygen concentration effect of gas bubbles adjacent to hot walls. If oxygen or air is absent, gum formation is not likely to occur.
In much of the prior art, the problems associated with gum and coke thermal deposits have been predominately dealt with by focusing on bulk fluid chemistry and reactions which can take place within the fluid. These investigations have involved a wide range of hydrocarbon compositions and the presence of numerous impurities such as sulfur compounds, nitrogen compounds, oxygen and trace metals. It has been observed that deposits attached to containment walls often contain very large quantities of sulfur and nitrogen compounds or intermediates thereof, in addition to gums and cokes. The prior art also discloses that some form of reaction takes place between the fuel and the wall. In U.S. Pat. No. 4,078,604, which is incorporated herein by reference, heat exchangers are characterized by thin-walled corrosion-resistant layers of electrodeposited gold or similar corrosion-resistant metals on the walls of the cooling channels in order to make the surfaces corrosion resistant to such storable liquid fuels as red fuming nitric acid. In this case, the wall is protected from corrosion, and the intent is not to prevent deposit formations. It is also known from the prior art that gold plating has been used to prevent surface catalytic reactions with fuel, in which case the objective is not to interfere with gum formation for the purpose of evaluating fuel chemical stability.
Thermal instability and fuel instability are becoming more significant with developing technology, and will become even more significant as processes and machinery are required to operate at higher temperatures as afforded by advances in materials technology and as the chemical quality of hydrocarbons for fuels, oils, lubricants, petrochemical processes (plastics and synthetics) and the like, decreases. Furthermore, hydrocarbon fluids, fuels and oils derived from non-petroleum sources, such as shale and coal, will have significantly more problems with thermal instability because of their high content of olefins, sulfur and other compounds. Accordingly, fluid containment articles that are resistant to or prevent the formation of adverse decomposition products and foulants are highly desirable in applications where thermal instability, including fuel instability, is a problem as a result of exposure to such fluids to high temperatures.
Articles and coatings having the above attributes are disclosed in U.S. Pat. Nos. 5,805,973 and 5,891,584, assigned to the assignee of the present invention, and incorporated herein by reference. These patents disclose the use of coatings that eliminate or reduce the surface reactions which lead to formation of thermal instability deposits from hydrocarbon fluids, and further eliminate or reduce adhesion of such deposits. These patents generally prevent the deposit on a metal surface of hydrocarbon degradation products derived from sulfur-containing hydrocarbon fluid by processing the metal surface to be clean and oxide-free, and then depositing a coating comprising a smooth catalytic layer of zirconium oxide and/or a diffusion barrier layer of a non-catalytic metal oxide, amorphous glass or mixtures thereof. The catalytic and non-catalytic layers are deposited by chemical vapor deposition from an undiluted organometallic precursor vapor. According to these inventions, the catalytic layer catalyzes thermal decomposition in the hydrocarbon fluid to promote the formation of coke, which is substantially non-adherent to the coating, while the non-catalytic diffusion barrier layer is inert to thermal decomposition in the hydrocarbon fluid and therefore inhibits the formation of gum, sulfur compounds and other decomposition impurities in the fluid.
While such coatings have been proven to be extremely effect, further reductions in the formation of gum and coke deposits on articles that must contain hydrocarbon fluids at high temperatures would be desirable.